FIG. 4 is a cross-sectional view showing a prior art semiconductor laser device. In FIG. 4, Reference numeral 1 designates an n type GaAs substrate. An n type Al.sub.0.25 Ga.sub.0.25 In.sub.0.5 P cladding layer 2 is disposed on the substrate 1. An undoped Ga.sub.0.5 In.sub.0.5 P active layer 3 is disposed on the cladding layer 2. A p type Al.sub.0.25 Ga.sub.0.25 In.sub.0.5 P cladding layer 4 is disposed on the active layer 3. An n type GaAs current blocking layer 5 is disposed on the cladding layer 4. A p type Ga.sub.0.5 In.sub.0.5 P buffer layer 6 is disposed on the cladding layer 4. A p type GaAs contact layer 8 is disposed on the current blocking layer 5 and the buffer layer 6. An n side electrode 10 is disposed on the rear surface of the substrate 1 and a p side electrode 11 is disposed on the contact layer 8. Reference numeral 9 designates a ridge approximately 3 to 6 microns in width. The distance between the current blocking layer 5 and the active layer 3 is approximately 0.3 microns. Respective layers 2 to 8 are crystal grown by metalorganic chemical vapor deposition (MOCVD).
The device will operate as follows.
When a forward direction voltage is applied between the p side electrode 11 and the n side electrode 10, a current concentratedly flows through the ridge portion 9 because the current blocking layers 5 are provided sandwiching the ridge portion 9. The n type Al.sub.0.25 Ga.sub.0.25 In.sub.0.5 P cladding layer 2, the Ga.sub.0.5 In.sub.0.5 P active layer 3, and the p type Al.sub.0.25 Ga.sub.0.25 In.sub.0.5 P cladding layer 4 constitute a double heterojunction structure, and laser light is generated in a region of the active layer 3 directly below the ridge portion 9. The p type Al.sub.0.25 Ga.sub.0.25 In.sub.0.5 P (energy band gap of 2.2 eV)/p type GaAs (energy band gap of 1.42 eV) heterojunction has a large energy band discontinuity therebetween and a large potential barrier exists in the valence band, and therefore a it is difficult for a current to flow through the heterojunction. Therefore, a p type Ga.sub.0.5 In.sub.0.5 P buffer layer 6 having an energy band gap of about 1.87 eV is inserted between the p type Al.sub.0.25 Ga.sub.0.25 In.sub.0.5 P cladding layer 4 and the p type GaAs contact layer 8 to relax the energy band discontinuity. If the buffer layer 6 is not provided, a predetermined laser light output power cannot be obtained unless the applied voltage is increased. The operating voltage can be reduced to a great extent by inserting the buffer layer 6.
The width of the ridge portion 9 mainly affects the transverse mode control. In this semiconductor laser device, since the distance between the active layer 3 and the current blocking layer 5 is about 0.3 micron and the thickness of the active layer 3 is below 0.1 micron, a portion of the light generated in the active layer 3 reaches the current blocking layer 5. However, since the energy band gap of the current blocking layer 5 is smaller than that of the active layer 3, a portion of the light generated in the active layer 3 is absorbed by the current blocking layer 5. On the other hand, no light absorption occurs in the ridge portion 9. Accordingly, a so-called "loss guide" (optical waveguide mechanism utilizing light absorption loss) is produced and a stable fundamental transverse mode is obtained by setting the ridge width at approximately 3 to 6 microns.
The p type Al.sub.0.25 Ga.sub.0.25 In.sub.0.5 P cladding layer 4 has a large resistivity of approximately 0.6 to 0.8 .OMEGA.cm irrespective of doping and the ridge width thereof is approximately 3 to 6 microns so as to control the transverse mode. Therefore, this cladding layer 4 is likely to have a high resistance and increases the operating voltage. In order to reduce the operating voltage, it is necessary to relax the band edge discontinuity between p type GaAs and p type AlGaInP.
In the prior art semiconductor laser device constituted as described above, the p type Ga.sub.0.5 In.sub.0.5 P buffer layer 6 is inserted between the p type Al.sub.0.25 Ga.sub.0.25 In.sub.0.5 P cladding layer 4 and the p type GaAs contact layer 8 to relax the band discontinuity between the p type AlGaInP and the p type GaAs, and to thereby reduce the operating voltage. However, even the buffer layer 6 is not sufficient.